The gas bag module of a vehicle occupant restraint system usually comprises a housing with a support piece for a gas generator and a covering for the gas bag which is held in the housing. The support piece and the covering may be constructed as separate components or may be formed integrally with further housing parts. High requirements are set with regard to the components of the gas bag module, owing to their being arranged visibly in the vehicle and their particular function of bearing loads or resisting high forces. For this reason, the housing parts such as coverings and gas generator supports have been produced up until now from metal or from a compactly injected plastic.
From DE 43 38 666 A1, an airbag housing is known which is produced in one piece by injection moulding. A pourable light metal alloy or a plastic which is able to be injection-moulded is used as the material for the airbag housing.
In DE 195 46 585 A1 a gas bag covering is described which has as a main component one or more layers of elastomer alloys which are joined to each other. The elastomer alloys consist of mixtures of a thermoplastic polymer with a non-cross-linked, partially cross-linked or fully cross-linked EPDM terpolymer, block copolymers of alternating polyester and polyether blocks or block copolymers of polystyrene and polyolefins. A tear line up to a predetermined residual wall thickness is introduced into the inner side of the covering by means of a laser.
Gas bag modules in which the covering is formed from a soft thermoplastic elastomer and the other housing parts consist of a hard plastic, for example a polyamide or polypropylene reinforced with glass fibres, are also commercially available.
A method for producing foamed thermoplastic elastomers is known from EP 1 090 065 B1.
However, the components of the gas bag module produced from a metal or from compactly injected plastics have a high weight. Furthermore, a certain inaccuracy in the components may occur with regard to the components produced from compactly injected plastics. Distortion and material shrinkage during the cooling of the molten plastic in the injection moulding tool are regarded as the cause of this inaccuracy in the components. Owing to differences in wall thickness in bending and hinged areas and also in the region of ribs and tear lines or other sudden changes in the wall thickness, deformations, shrink marks and blemishes therefore appear on the surface which are particularly not desired in gas bag module coverings and require a reworking of the workpiece.